1. Technical Field
The present invention relates to a brightening ink composition for forming, by gravure printing, a printed layer which imparts design applicability and aesthetic properties to the surface of a heat insulating foamed container, a paper container material using the ink composition, and a heat insulating foamed paper container. More particularly, the present invention relates to a brightening ink composition for forming a printed layer which has excellent foaming adaptability, has less surface unevenness, and provides a smooth surface with suppressed occurrence of cracks in the ink film, on the film of a paper container material having a thermoplastic synthetic resin film that is foamed by a heat treatment carried out upon the production of a heat insulating foamed paper container and thereby forms a heat insulating layer; a paper container material using the ink composition; and a heat insulating foamed paper container. The heat insulating foamed paper container having a printed layer that is formed from the brightening ink composition of the present invention has a smooth surface and exhibits excellent heat insulating properties, and therefore, the container can be suitably used as a container for holding food containing a high-temperature liquid or low-temperature liquid. For example, the heat insulating foamed paper container according to the present invention can be suitably used as a container such as a cup or a bowl for foods including soup, sweet red bean soup, miso soup and noodles.
2. Background Art
In recent years, the demands for ink and the printing industry are extending into a broad range of applications, concomitantly with the varietization of packaging containers. For example, in the field of containers such as the cups used for those food products generally called as “cup noodles,” including ramen products, udon and soba noodles, which become edible several minutes after an appropriate amount of boiling water is poured into the container, polystyrene cups have been mainly used from the viewpoint of cost and heat insulating properties.
However, in the case of foamed polystyrene cups, printing on the cups must be performed individually after the forming of cups. Accordingly, the printing method is limited to methods of using a curved surface printing machine or a stamp printing machine, and there has been a problem that the printing speed and the print quality are poor. Furthermore, owing to the reasons such as that the polystyrene price is increasing concomitantly with a recent increase in the price of petroleum, and that under the implementation of the Containers and Packaging Recycling Law, the food product manufacturers are responsible for the recycling fee, transition from polystyrene cups to paper cups is being promoted in recent years.
As one of the representative structures of paper containers, there is known a double-walled cup having a heat insulating air layer which is formed by bonding paper and paper with furrows therebetween. However, the double-walled cup has problems in terms of high weight and high price. Therefore, heat insulating foamed paper containers are attracting more attention these years.
A heat insulating foamed paper container is generally produced using a paper container material having a thermoplastic synthetic resin film that is foamed by a heat treatment carried out upon production of the container and forms a heat insulating layer. More specifically, the paper container material has a structure in which, for example, a high melting point polyethylene film having a melting point of about 130° C. to 135° C. is laminated on one surface of a base paper (inner side of container), and a low melting point polyethylene film having a melting point of about 105° C. to 110° C. (hereinafter, referred to as “low-Mp film”) is laminated on the other surface of the base paper (outer side of container), and a printed layer containing print patterns such as decorative patterns, the company name and a barcode is formed on the surface of the low-Mp film.
In the production process for the heat insulating foamed paper container, a paper container material having a print layer formed in advance is punched out into a predetermined shape, this is used as a body member of cup to form a cup, and then the cup is subjected to a heat treatment at near the melting point of the tow-Mp film, thereby the low-Mp film being foamed. Foaming occurs when the moisture contained in the base paper evaporates at the time of the heat treatment and is pushed out to the side of the low-Mp film which has been softened, and thereby the low-Mp film expands toward the outer side. The low-Mp film thus foamed functions as a heat insulating layer and imparts heat insulating properties to the paper container. Such a heat insulating foamed paper container is disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. H9-95368 (Patent Document 1), JP-A No. H7-232774 (Patent Document 2), and JP-A No. H11-189279 (Patent Document 3).
The formation of a printed layer on the surface of the low-Mp film of the material for heat insulating foamed paper containers is usually carried out according to a surface printing method based on gravure printing. In the field of surface printing based on gravure printing, ink is applied on the surface of objects to be printed, such as various base papers and plastic packaging containers. Therefore, inks for gravure printing are requested to have printing suitability for objects to be printed, as well as adhesiveness to the base papers and plastic materials, and various resistances such as the resistance to blocking to prevent adhesion between printed matters after printing, and the abrasion resistance and heat resistance required upon the formation of container.
Furthermore, an ink for gravure printing usually contains an aromatic solvent such as toluene as a main solvent, so as to achieve a balance between the solubility of binder resins and the dryability. However, recently, the tendency toward using a printing ink that does not contain an aromatic solvent is growing stronger, in order to improve the environment of the printing worksite.
Moreover, when the product value as a heat insulating foamed paper container is considered, the ink for gravure printing for forming a printed layer of the container is desirably such that the ink film does not impede but promotes foaming of the low-Mp film that constitutes the heat insulating layer and can thereby provide a container surface having less surface unevenness, and that the ink film has less occurrence of cracks and can provide a smooth printed surface.
As the ink for gravure printing for forming printed layers of heat insulating foamed paper containers, there have been traditionally known inks containing polyamide resins and cellulose derivatives as binder components. However, in the case of an ink containing a polyamide resin as a main component of the binder components, when the ink film is exposed to heat or light over a long time, the low molecular weight components such as oils and fats contained in the polyamide resin undergo oxidative degradation into acetaldehydes, thereby causing a problem of generating a fat-based odor. Furthermore, since there is a possibility that the content in the container may cause a change in the taste due to the generated fat-based odor, this poses a serious problem.
Furthermore, when the printing ink described above is prepared as a brightening ink containing a brightening material such as an aluminum material, and is used to form a printed layer, the ink film tends to markedly suppress the foaming of the low-Mp film. Therefore, when a printed layer formed from a brightening ink has been directly provided on the low-Mp film, conspicuous surface unevenness occurs between the printed areas and the non-printed areas, and thus it is difficult to obtain a smooth printed surface. Accordingly, in the case of forming a printed layer using a brightening ink, usually an undercoat layer of a white ink is provided on the low-Mp film. However, brightening inks have a large suppressive power against the foaming of the low-Mp film as compared with white inks. Thus, even when a white ink is printed as an undercoat layer and a brightening ink is overprinted thereon, it is difficult to make uniform of the thickness of the low-Mp film after foaming.
The surface of a container having such a printed layer is such that the printed areas of white ink/brightening ink overprinting are obviously depressed as compared with the printed areas of white ink, and clear differences in level occur. Therefore, the heat insulating foamed paper container does not acquire a smooth feeling of touch, and have problems such as impaired merchantability or unreadable barcodes. Furthermore, since the foaming of the low-Mp film is suppressed, a heat insulating layer having a sufficient thickness is not obtained, and there are also problems in terms of heat insulating properties, such as the container becomes hot when hot water is placed in the container.
On the other hand, JP-A No. 2006-218708 (Patent Document 4) discloses a colored ink which is applicable to gravure printing, has a sense of glitter, and is excellent in high glossiness and design applicability. The disclosed colored ink contains a pearl pigment and silica as brightening materials, and is mainly used for forming an undercoat layer of glossy decorative papers that are applied to the interior design materials of construction materials and the like.
Patent Document 1: JP-A No. H9-95368
Patent Document 2: JP-A No. H7-232774
Patent Document 3: JP-A No. H11-189279
Patent Document 4: JP-A No. 2006-218708